1. Field of the Invention
This invention relates in general to an apparatus for fabricating a soot preform for an optical fiber, and more specifically relates to an apparatus for stably fabricating a high-quality soot preform for an optical fiber by vapor-phase axial deposition.
2. Description of Related Art
Vapor-phase axial deposition (VAD) is a well-known process for fabricating a fiber preform nowadays. A starting rod is installed on a shaft capable of being rotated and shifted vertically into and out of a reaction chamber. Glass particles generated by a core burner and a cladding burner in the reaction chamber are deposited on the front end of the starting rod, thereby a porous soot preform (soot preform, hereinafter) consisting of a core and a cladding layer for an optical fiber is fabricated.
In general, the deposition efficiency for depositing the glass particles on the starting rod is not 100%, a lot of stray glass particles, which are not adhered or deposited on the starting rod, occur during the fabrication. Most of the stray glass particles are exhausted from an exhausting pipe of the reaction chamber along with other gases that should be exhausted. However, a portion of the stray glass particles are adhered to a ceiling and sidewalls of the reaction chamber during exhaustion of stray glass particles and other gases.
In general, the glass particles are generated by injecting chlorides serving as a source gas (such as SiCl4) into an oxyhydrogen flame to thereby effect a flame hydrolysis process. The gases, such as water vapor and hydrochloric acid that are generated in this process and ought to be exhausted as well as the stray glass particles, which are not adhered and accumulated on the starting rod, are at high temperatures and tend to enter an upper room assembled on top of the reaction chamber.
Because the temperature of the upper room is not as high as the temperature of the reaction chamber, water vapor entering the upper room is condensed on the inner wall of the upper room and the hydrochloric acid is absorbed by it. Therefore, the upper room is eroded if the upper room is made of metals. Even if the upper room is made of erosion-resistant materials, the apparatus is difficult to clean up after the preform fabricating process is finished. Furthermore, the stray glass particles adhere to the hydrochloric acid-wet inner walls of the reaction room, resulting in that the apparatus is still more difficult to clean up.
Additionally, the stray glass particles, which first failed to adhere on the preform but later deposit on the surface of an off-the-frame portion of the preform, grow finely on the soot preform like trees. Then, in the subsequent glassification process, the tree-like protrusions are formed on the surface of the soot preform, causing difficulty in measuring the distribution of refraction index.
In general, for avoiding foregoing issues, a downward gas flow from the top of the upper room to the reaction chamber is used, thereby preventing the stray glass particles from adhering and accumulating on the walls of the upper room to a degree, but cannot this could not prevent the particles from adhering and accumulating to the ceiling and the walls of the reaction chamber.
In the conventional methods, in the post stage of the fabrication of the soot preform, flakes of the glass particles adhered or accumulated on the inner walls of the reaction chamber get detached from the walls and fell, stirring up the glass particles, some of which fell on the soot preform, causing that bubbles and impurities are formed in the glassified soot preform.
Recently, because the fiber demand is increased and its cost is requested to be reduced, it is very important to enlarge the optical fiber preform. Naturally , material supply must be increased for enlarging the fiber preform. Once the material supply is increased, the amount of the stray glass particles increase even if the deposition efficiency does not changed. Therefore, in the conventional methods, the frequency of the falling in masses of the stray glass particles from the inner walls of the reaction chamber increased.
In order to enhance the exhausting efficiency of the stray glass particles, a method was proposed to increase the gas-supplying amount and the gas-exhausting amount. However, this method causes the gas flow in the reaction chamber to be more turbulent, and the flame of the core burner, whose gas flow rate is relative low, is seriously disturbed. As a result, the distribution of refraction index of the soot preform in the length-wise direction becomes uneven.
Furthermore, if the radius of the soot preform is large, the gap between the soot preform and the wall of the upper room changes drastically as the soot preform is gradually moved into the upper room, especially which the tapered top of the preform passes the boundary between the upper room and the reaction chamber. Therefore, at the boundary between the reaction chamber and the upper room, i.e., the entrance of the reaction chamber, the gas flow from the upper room to the reaction chamber is very fast after the trunk of the soot preform is moved into the upper room.
The gas flow, adjusted as of the beginning of the fabrication to properly prevent the gases and the stray glass particles from entering the upper room, becomes so strong when the preform starts entering the upper room that it disturbs the core deposition.
For solving the foregoing problems, Japanese Laid Open 9-118537 and 11-343135 provide apparatuses capable of effectively exhausting the stray glass particles that are not properly deposited on the soot preform.
According to Japanese Laid Open 11-343135, it comprises two reaction chambers: one is for depositing core and the other is for depositing cladding. Each of the reaction chambers is equipped with an exhausting damper capable of adjusting exhausting pressure respectively. The exhausting pressure for the separated cladding reaction chamber is set higher than that used in the conventional reaction chamber so that even when the exhausting amount from the cladding reaction chamber is increased, the flame for depositing the core is not disturbed. Therefore, the soot preform can be fabricated stably.
However, because the reaction chamber is divided into two separated reaction chambers, the control of the exhausting pressure for each reaction chamber becomes difficult. In addition, the glass particles generated during the core deposition tend to accumulate on the lower side face of the partition, leading to the same drawbacks described above.
According to Japanese Laid Open 11-343135, gas is introduced through a whole sidewall behind a burner in the reaction chamber and a gas-exhausting outlet is installed in a sidewall opposite to the sidewall through which the gas is introduced. Furthermore, a flow-guide wall having numerous gas blowout holes is provided to each of the two sidewalls, between which the soot preform poses, thereby the amount of the glass particles adhered or accumulated on the inner walls of the reaction chamber is reduced.
However in an apparatus as this, the gas flow around the core burner becomes faster and more turbulent, thereby the core deposition is disturbed and an even distribution of refraction index in the lengthwise direction cannot be obtained.
In addition, according to FIG. 1 of Japanese Laid Open 11-343135, for preventing the air from entering the upper room through gaps between an upper cap and a driving shaft, a seal gas is introduced into the upper room from its top. However, it could not efficiently prevent the exhausting gases and the stray glass particles from entering the upper room.